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United States Patent |
5,687,111
|
Wada
,   et al.
|
November 11, 1997
|
Static type semiconductor memory device capable of operating at a low
voltage and reducing a memory cell area
Abstract
A pair of driving bipolar transistors of a lateral type T1 and T2 have
emitters coupled to a ground potential, collectors connected to a pair of
highly resistive elements R1 and R2. Highly resistive elements R1 and R2
have respective other ends coupled to power supply potential V.sub.CC, and
bases and collectors of transistors T1 and T2 are cross-connected to each
other, thereby forming a flipflop circuit. Access MOS transistors Q3 and
Q4 having a gate potential controlled by word line WL are each connected
to form a conduction path between one of storage nodes A and B and one of
the pair of bit lines BL and /BL.
Inventors:
|
Wada; Tomohisa (Hyogo, JP);
Kozaru; Kunihiko (Hyogo, JP);
Shiomi; Toru (Hyogo, JP)
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Assignee:
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Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)
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Appl. No.:
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609313 |
Filed:
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March 1, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
365/154; 257/378; 257/903; 257/E27.098; 257/E27.099; 257/E27.1; 257/E27.101; 365/174; 365/177; 365/182 |
Intern'l Class: |
G11C 011/00; H01L 027/02 |
Field of Search: |
365/154,174,177,182
257/903,378
|
References Cited
U.S. Patent Documents
3986173 | Oct., 1976 | Baitinger et al. | 365/154.
|
4661831 | Apr., 1987 | Schmitt-Landsiedel et al. | 365/154.
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5404030 | Apr., 1995 | Kim et al. | 257/903.
|
5453636 | Sep., 1995 | Eitan et al. | 365/154.
|
Foreign Patent Documents |
62-254463 | Nov., 1987 | JP.
| |
2-237151 | Sep., 1990 | JP.
| |
3-83293 | Apr., 1991 | JP.
| |
3-234059 | Oct., 1991 | JP.
| |
Other References
"A 34ns 1-Mbit CMOS SRAM Using Triple Polysilicon" by Tomohisa WADA et al.,
IEEE, 1987.
"A 9-ns 1 Mbit CMOS SRAM" by Katsuro Sasaki et al., IEEE, 1989.
|
Primary Examiner: Dinh; Son T.
Attorney, Agent or Firm: Lowe, Price, LeBlanc & Becker
Claims
What is claimed is:
1. A static type semiconductor memory device including memory cells each
having a flipflop circuit in which inputs and outputs of two inverters are
coupled to each other, said device comprising:
a semiconductor substrate having a main surface;
a transistor group having multiple sets of a pair of access MOS transistors
and a corresponding pair of driving bipolar transistors of a lateral type
formed at said main surface of said semiconductor substrate;
an interlayer insulating layer covering a surface of said transistor group;
and
a pair of load elements formed on said interlayer insulating layer
corresponding to each pair of said driving bipolar transistors of a
lateral type; wherein
each of said inverters includes said load element and said driving bipolar
transistor of a lateral type;
said load element has one end connected to a first power supply and another
end connected to a collector of said driving bipolar transistor of a
lateral type, and
said driving bipolar transistor of a lateral type has an emitter region and
a collector region separately formed on said main surface of said
semiconductor substrate;
a base region of said driving bipolar transistor of a lateral type is
formed on said main surface of said semiconductor substrate and surrounds
completely said emitter region and collector region except said main
surface of said semiconductor substrate;
emitters of said driving bipolar transistor of a lateral type are connected
to a second power supply;
collectors and bases of said pair of driving bipolar transistors of a
lateral type are cross-connected to each other; and
a base region of said driving bipolar transistor of a lateral type has a
common region shared with one of source and drain regions of said access
MOS transistor.
2. The static type semiconductor memory device according to claim 1,
wherein said driving bipolar transistor of a lateral type has, in the
semiconductor substrate under said base region, a buried high
concentration layer of a same conductivity type as the base region, having
an impurity concentration higher than that of the base region.
3. The static type semiconductor memory device according to claim 1,
wherein said driving bipolar transistor of a lateral type has a
non-conductive buried layer in the semiconductor substrate under said base
region.
4. The static type semiconductor memory device according to claim 1,
wherein said common region is different from said base region in terms of
at least one of impurity concentration and depth.
5. A static type semiconductor memory device including memory cells each
having a flipflop circuit in which inputs and outputs of two inverters are
coupled to each other, said device comprising:
a semiconductor substrate having a main surface;
a transistor group having multiple sets of a pair of access MOS transistors
and a corresponding pair of driving bipolar transistors of a lateral type
formed at said main surface of said semiconductor substrate;
an interlayer insulating layer covering a surface of said transistor group;
and
a pair of load elements formed on said interlayer insulating layer
corresponding to each pair of said driving bipolar transistors of a
lateral type; wherein
each of said inverters includes said load element and said driving bipolar
transistor of a lateral type,
said load element has one end connected to a first power supply and another
end connected to a collector of said driving bipolar transistor of a
lateral type, and said driving bipolar transistor of a lateral type has an
emitter connected to a second power supply,
collectors and bases of said pair of driving bipolar transistors of a
lateral type are cross-connected to each other,
a base region of said driving bipolar transistor of a lateral type has a
common region shared with one of source and drain regions of said access
MOS transistor,
said common region is different from said base region in terms of at least
one of impurity concentration and depth, and
said driving bipolar transistor of a lateral type has, in the semiconductor
substrate under the base region, a buried high concentration layer of a
same conductivity type as the base region, having an impurity
concentration higher than that of the base region.
6. A static type semiconductor memory device formed on an SOI substrate
having a thin film semiconductor layer on an insulator as a main surface,
and including memory cells each having a flipflop circuit having two
inverters, said device comprising:
a transistor group having multiple sets of a pair of access thin film MOS
transistors and a pair of driving bipolar transistors of a lateral type
formed at said thin film semiconductor layer;
an interlayer insulating layer covering a surface of said transistor group;
and
a pair of load elements formed on said interlayer insulating layer
corresponding to each pair of said driving bipolar transistors of a
lateral type, and connected to said driving bipolar transistors of a
lateral type through contact holes formed at said interlayer insulating
layer; wherein
each of said inverters includes said load element and said driving bipolar
transistor of a lateral type;
said load element has one end connected to a first power supply and another
end connected to a collector of said driving bipolar transistor of a
lateral type, and said driving bipolar transistor of a lateral type has an
emitter connected to a second power supply;
collectors and bases of said pair of driving bipolar transistors of a
lateral type are cross-connected to each other;
a base region of said driving bipolar transistor of a lateral type has a
common region shared with one of source and drain regions of said access
thin film MOS transistor;
a collector region of said driving bipolar transistor of a lateral type is
formed by a region of a same conductivity type extending from a surface of
said thin film semiconductor layer to an interface with said insulator;
and
said contact hole is open to the collector region of one transistor and the
base region of the other transistor of said pair of driving bipolar
transistors of a lateral type.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor memory devices, and more
particularly, to circuit arrangement and structure of a static type RAM
capable of operating at a low voltage.
2. Description of the Background Art
In a static type RAM, a memory cell is formed by a flipflop circuit and an
access transistor. As the structure of such a memory cell, a highly
resistive load type and a CMOS type are known. The highly resistive load
cell is advantageous in enhancing integration level since a resistor can
be stacked on a transistor, but has a disadvantage that it lacks stability
in holding data. The CMOS type cell excels in stability in holding data as
compared with the highly resistive load type cell, but has a disadvantage
that greater area is occupied on a substrate. Thus, a technique for
forming a cell by using a TFT (Thin Film Transistor) as a load is
implemented in order to utilize the characteristics of the CMOS type cell
and maintain the high integration level.
FIG. 19 is an equivalent circuit diagram of a memory cell in a conventional
static type semiconductor memory device of a highly resistive load type.
N type MOSFETs Q1 and Q2 and highly resistive elements R1 and R2 are
connected respectively in series between a power supply potential V.sub.CC
and a ground potential Vss, forming two inverters. Respective output nodes
A and B of the inverters are cross-connected with gates of N type MOSFETs
Q1 and Q2, forming one flipflop circuit. Nodes A and B function as storage
nodes of the memory cell.
Each of N type MOSFETs Q3 and Q4, which are access transistors, has
source/drain terminals connected between one of the pair of bit lines BL
and /BL and one of the pair of storage nodes A and B of the memory cell,
and a gate connected to a common word line WL.
Usually, for reduction in memory cell area, transistors Q1, Q2, Q3 and Q4
of the same conductivity type forming the memory cell are formed at a
surface of a silicon substrate, and highly resistive load elements R1 and
R2 are formed thereon with an interlayer insulating film interposed
therebetween.
An example of the memory cell in which a highly resistive load element is
formed at a layer over a transistor as described above is disclosed in an
article by T. Wada et al., "A 34-ns 1-Mbit CMOS SRAM Using Triple
Polysilicon", IEEE Journal of Solid State Circuits, Vol-SC22, No. 5,
October 1987, pp. 727-732.
FIG. 20 shows a memory cell of a CMOS type in which the resistive load in
FIG. 19 is replaced by a P type MOSFET. If the P type MOSFET is formed by
a thin film transistor (TFT) and arranged over N type MOSFETs Q1, Q2, Q3
and Q4, this memory cell can also be implemented in substantially the same
memory cell area as the memory cell of a highly resistive load type shown
in FIG. 19.
An example of a memory cell utilizing a TFT transistor is disclosed in an
article by K. Sakai et al., "A 9-ns 1-Mbit CMOS SRAM", IEEE Journal of
Solid State Circuits, Vol-24, No. 5, October 1989, pp. 1219-1225.
Problems of a memory cell in a conventional static type semiconductor
memory device will be described below by taking a memory cell of a highly
resistive load type as an example.
FIG. 21 shows a layout of the memory cell shown in the equivalent circuit
diagram of FIG. 19. Since the mask pattern for forming the memory cell in
FIG. 19 consists of many layers, the layers thereof are divided as shown
in FIGS. 21(a), (b) and (c) for illustration of the whole structure. The
layers shown in FIGS. 21(a), (b) and (c) actually overlap with each other.
Referring to FIG. 21(a), portions, where active layer regions 31a and 3lb
formed at the surface of the silicon substrate are overlapped with first
polysilicon patterns 32a and 32b, correspond to channel regions of driving
N type MOSFETs Q1 and Q2 shown in FIG. 19, respectively. First polysilicon
patterns 32a and 32b correspond to the gates of N type MOSFETs Q1 and Q2.
Portions where active layer regions 31a and 31b are overlapped with first
polysilicon patterns 32c and 32d correspond to channel regions of the
access transistors, N type MOSFETs Q3 and Q4, shown in FIG. 19,
respectively.
First polysilicon patterns 32c and 32d corresponding to the gates of the
access transistor N type MOSFETs Q3 and Q4 correspond to word line WL in
FIG. 19. First buried contacts 41a and 41b are formed by opening contact
holes at a gate oxide film formed at the surface of the silicon active
layer, through which contacts first polysilicon patterns 32a and 32b are
connected to the surface of the silicon substrate. These connections form
the cross-connections of the gates and drains of N type MOSFETs Q1 and Q2
shown in FIG. 19.
Regarding the pattern formed at a layer over this pattern, a second
polysilicon pattern 43 is used as an interconnection for connecting the
memory cell to the ground, as shown in FIG. 21(b). Second polysilicon
pattern 43 is connected to the surface of the silicon substrate through
second buried contacts 42a and 42b, thereby forming interconnections for
connecting sources of N type MOSFETs Q1 and Q2 in FIG. 19 to the ground.
Third polysilicon patterns 44a and 44b function as interconnections for
connecting resistors R1 and R2 in FIG. 19 and connecting these resistors
and power supply potential V.sub.CC.
Regarding a further upper pattern, with reference to FIG. 21(c), ions are
implanted to third polysilicon patterns 44a and 44b by using a mask
pattern 46 as a mask to reduce interconnection resistance at the unmasked
portion. Portions where mask pattern 46 is overlapped with third
polysilicon pattern 44 become highly resistive polysilicon, serving as
load resistors R1 and R2 of the memory cell. Metal interconnections 36a
and 36b correspond to bit line pair BL and /BL and connected to the
surface of the silicon substrate to contact holes 35a and 35b. As a
result, bit line pair BL and /BL and access transistors Q3 and Q4 are
connected respectively.
As a metal interconnection, an Al--Si interconnection, an Al--Si--Cu
interconnection or the like is used.
FIG. 22 is a cross sectional view taken along the lines A-A' and B-B' in
the patterns shown in FIG. 21 (the mask patterns in the lithography
process of manufacturing a semiconductor device are formed based on these
patterns). A field oxide film 52 for element isolation is formed on a
silicon substrate 51. A portion of a silicon substrate 51 which is not
covered by field oxide film 52 acts as a silicon active region. An access
transistor is formed by N.sup.+ source/drain regions 54a and 54b and
first polysilicon gate layer 32c formed on the silicon active layer with
the gate oxide film interposed therebetween, and a portion immediately
under first polysilicon gate layer 32c serves as a channel portion of the
access transistor. In contrast, first polysilicon layer 32d is a mere
interconnection since it is located on field oxide film 52 in this cross
section. Second polysilicon pattern 43 applies the ground potential. Third
polysilicon pattern 44b in FIG. 21 corresponds to two regions of
interconnection portion 69 having a reduced resistance and a load
resistance portion 70 having a increased resistance by ion implantation.
Bit line 36a is a metal interconnection and connected to N.sup.+
source/drain region 54a through contact hole.
Next, operations of the highly resistive load type memory cell shown in
FIG. 19 will be described. The following description is made by placing a
special emphasis on stability of the memory cell operations at a low
voltage. It is assumed that storage node B is at an "H" (High) level in a
stand-by state.
Since access transistors Q3 and Q4 in the memory cell are turned off in the
stand-by state, the inverters of the memory cell are formed by driver
transistors Q1 and Q2 and highly resistive loads R1 and R2 and have a high
voltage gain. In other words, an output of the inverter makes a steep
transition near the logic threshold value. In this case, noise margin is
considerably large and data is stably held.
During data readout, access transistors Q3 and Q4 in the memory cell are
turned on whereby a current flows from/bit line /BL to storage node B at
an "L" (Low) level. More specifically, this is equivalent to the state
where a load with a low impedance is connected in parallel to a load
element and no resistive load with a high impedance is present. Therefore,
the inverter in the memory cell must be treated as an N type enhancement
MOS load type having an access transistor Q3 or Q4 as a load. At this
time, the gain of the inverter is considerably reduced from that in the
stand-by state and the output of the inverter makes a more gradual
transition. This is the most dangerous moment for the memory cell when a
bistable state is lost and data is destroyed unless it has a sufficient
noise margin.
In order to suppress the consumed current sufficiently in the stand-by
state, the resistance of the highly resistive load element is a
sufficiently great value such as approximately 10 T.OMEGA. and the
potential of the storage node corresponding to an "H" level immediately
after writing is at a level of V.sub.CC -V.sub.thn, a level lower than the
power supply voltage by the threshold voltage (V.sub.thn) of the access
transistor. Therefore, analysis of bistability of a memory cell must be
carried out for the readout operation immediately after writing when the
operation margin is the smallest.
Readout characteristics of the memory cell immediately after writing is
shown in FIG. 23(a) and the circuit structure of the memory cell
corresponding to the readout characteristics is shown in FIG. 23(b). A
curve .alpha. in FIG. 23(a) indicates the characteristics of an inverter
formed by access transistor Q2 and driver transistor Q4, and a curve
.beta. indicates the characteristics of an inverter formed by access
transistor Q1 and driver transistor Q3. The effects of resistive loads R1
and R2 can be ignored since a current flowing therethrough is small, as
described above.
Crossing points a and b of curves .alpha. and .beta. are the stable points
of the memory cell, and point a corresponds to the case where data of "0"
is stored and point b corresponds to the case where data of "1" is stored.
A portion C in FIG. 23(a) corresponds to a threshold voltage V.sub.athn of
access transistor Q4, and a portion D corresponds to a threshold voltage
V.sub.athn of driver transistor Q1. The remaining portion E obtained by
subtracting portions C and D from power supply voltage V.sub.CC
corresponds to a margin region for stabilizing readout operations of the
memory cell. It is difficult to reduce threshold voltages of an access
transistor and a driver transistor to a certain value or lower, since the
subthreshold leakage current of these transistors must be suppressed
sufficiently. Therefore, margin region E is decreased with lower operation
voltage V.sub.CC, whereby readout operations are not stabilized.
Portions indicated by p and q in FIG. 23(a) are called an "eye of a cell",
and a region corresponding to the eye of a cell is smaller where the power
supply voltage is lower, and operations are instabilized.
Therefore, in order to improve the operation margin of the memory cell
during low voltage operations as much as possible, a ratio (cell ratio) of
the current supplying capabilities of the driver transistor to the access
transistor is sufficiently increased.
More specifically, when an "H" level is stored in storage node A and the
current supplying capability of driver transistor Q2 is greater than that
of access transistor Q4 which transistors are connected to storage node B
where an "L" level is stored, then the potential of storage node B is
stabilized at an "L" level. This means that the value indicated by F in
FIG. 23(a) is decreased.
Here, the current supplying capability of an MOS transistor is generally
expressed by the value of .beta. in the following equation:
I.sub.D =.beta.(V.sub.GS -V.sub.th).sup.2 /2 (1)
where I.sub.D indicates a drain current in a saturation region, V.sub.GS
indicates a voltage between the gate and the source and V.sub.th indicates
the threshold voltage of the transistor.
The cell ratio has been set at a value of 3 or more with the operation
margin in view.
However, the value .beta. is proportional to W/L where W is a channel width
and L is a channel length of a transistor. Thus, setting the cell ratio at
a value of 3 or more leads to greater areas of driver transistors Q3 and
Q4, thereby preventing reduction in area of the memory cell.
Description will now be made of a second conventional example for enhancing
stability in operation of the memory cell at a low voltage.
FIG. 24 is a circuit diagram showing an equivalent circuit of a memory cell
in the second conventional example. It is different from the first example
shown in FIG. 19 in that N type MOSFETs used as access transistors Q3 and
Q4 are replaced by P type MOSFETs. Since a P type MOSFET is utilized as an
access transistor in this example, the word line obtains an "L" level when
selected.
To clarify illustration of operations of the memory cell in the second
example shown in FIG. 24, the memory cell is divided into two inverters as
shown in FIGS. 25(a) and (b). The potential of storage node A will be
hereinafter indicated by V(2) and the potential of storage node B
indicated by V(1). When the memory cell is in a readout state, i.e. access
transistor Q3 or Q4 is turned on, effects of load resistive elements R1
and R2 can be neglected as stated in the first example. Therefore, the
memory cell inverter must be treated as the CMOS type having access
transistor Q3 or Q4 as a load.
Since access transistors Q3 and Q4 are N type MOSFETs in the first example,
operation power supply voltage Vcc is required to satisfy the following
relationship:
V.sub.CC >V.sub.athn +V.sub.dthn ( 2)
More specifically, the relationship above must be satisfied in order to
secure margin region E shown in FIG. 23. For example, assuming that the
driver transistor has threshold voltage V.sub.dthn of 0.8 V and the access
transistor has threshold voltage V.sub.athn of 1.3 V taking into account a
rise in the threshold voltage due to back gate effects, then V.sub.CC is
required to exceed 2.1 V. Therefore, it is difficult to operate this
memory cell at a low voltage.
With the structure of the second example shown in FIG. 24, however, voltage
drop by the threshold voltage of the transistor can be neglected since
access transistors Q3 and Q4 are P type MOSFETs, and thus operation power
supply voltage Vcc is required only to satisfy the following relationship:
V.sub.CC >V.sub.dthn ( 3)
If V.sub.dthn is, for example, 0.8 V, the memory cell can operate at as
small a voltage as V.sub.CC =1 V.
FIG. 26 plots the transfer characteristics of two inverters forming the
memory cell in the selected state shown in FIG. 25 to a V(1)-V(2) plane.
It should be noted that all the characteristics shown in FIG. 26 are
obtained when the cell ratio is equal to 1.
The crossing point of the transfer characteristics of the two inverters is
the stable point of the memory cell. Note that the crossing point in the
middle is a unstable point and crossing points a and b shown in the left
upper portion and right lower portion of the graph are the stable points.
The three kinds of curves in FIG. 26 correspond to the cases where
V.sub.CC is equal to 1 V, 2 V, and 3 V, respectively, and two stable
points are present for each case. The "eyes of a cell" formed by the
transfer characteristic curves of the inverters at respective power supply
voltage conditions indicate the static noise margin of the bistable
circuit. The stability thereof is enhanced as the margin is increased. The
results of the simulation shown in FIG. 26 suggest that the memory cell
can operate at a voltage V.sub.CC of 1 V.
Therefore, with such a structure as in the second example, the condition
requiring the cell ratio of 3 or more in the first example need not be
satisfied, so that the access transistor and the driver transistor can be
substantially equal in size.
However, in this case as well, an N type MOSFET and a P type MOSFET must be
formed at the same time in the same memory cell, leading to an undesirable
increase in area of the isolation region for element isolation.
Since the conventional static type semiconductor memory device has a memory
cell with a structure as described above, it has the following problems.
First, if the access transistor and the driver transistor are both N type
MOSFETs, the driver transistor must be increased in size to achieve low
voltage operations, making it difficult to reduce the area of the memory
cell. In addition, even if the cell ratio is increased, there is a limit
to low voltage operations.
Secondly, if the access transistor is formed as a P type MOSFET to achieve
low voltage operations, the area of an isolation region between elements
is increased, thereby preventing reduction in area of the memory cell.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a static type
semiconductor memory device including a memory cell having a sufficient
operation margin even when a power supply voltage is low.
Another object of the present invention is to provide a statistic type
semiconductor memory device capable of reducing memory cell area and
achieving low voltage operations.
Briefly stated, the present invention relates to a static type
semiconductor memory device which includes a memory cell array having a
plurality of word lines, a plurality of bit line pairs crossing the
plurality of word lines and a plurality of memory cells connected to the
word lines and the bit line pairs. Each memory cell includes a pair of
bipolar transistors having base regions of a first conductivity type with
their emitters connected to first power supplies and bases and collectors
are cross-connected to each other, a pair of load elements connected
between second power supplies and the collectors of the pair of bipolar
transistors, and a pair of first conductivity type channel MOS transistors
which have gates connected to the word line and are each connected to form
a conduction path between the collector of one of the pair of bipolar
transistors and the corresponding bit line.
According to another aspect of the present invention, a static type
semiconductor memory device, including a memory cell formed by a flipflop
circuit in which inputs and outputs of two inverters are coupled to each
other, is provided which includes a semiconductor substrate, a group of
transistors, an interlayer insulating layer, and a plurality of load
elements. The semiconductor substrate has a main surface. The group of
transistors has plural sets of a pair of access MOS transistors and a
corresponding pair of driving bipolar transistors of a lateral type formed
at the main surface of the semiconductor substrate. The interlayer
insulating layer covers a surface of the transistor group. Load elements
form a pair corresponding to each pair of the driving bipolar transistors
of a lateral type and are formed on the interlayer insulating layer. The
load element has one end connected to a first power supply and another end
connected to the driving bipolar transistor of a lateral type, and the
driving bipolar transistor of a lateral type has an emitter connected to a
second power supply, so that each load element and the corresponding
driving bipolar transistor of a lateral type form an inverter. The pair of
driving bipolar transistors of a lateral type have collectors and bases
which are cross-connected to each other, and base regions each having a
common region shared with one of source/drain regions of the access MOS
transistor.
According to still another aspect of the present invention, a static type
semiconductor memory device having a thin film semiconductor layer on an
insulator as a main surface and formed on an SOI substrate is provided
which includes a memory cell array having a plurality of word lines, a
plurality of bit line pairs crossing the plurality of word lines, and a
plurality of memory cells connected to the word lines and the bit line
pairs. Each memory cell includes a pair of bipolar transistors formed at
the thin film semiconductor layer and having emitters connected to first
power supplies, bases and collectors cross-connected to each other, and
base regions of a first conductivity type; a pair of load elements
connected to form conduction paths between second power supplies and the
collectors of the pair of bipolar transistors; and a pair of first
conductivity type channel MOS transistors formed at the thin film
semiconductor layer and having gates connected to the word line, and
connected between the collectors of the pair of bipolar transistors and
the bit lines.
According to a further aspect of the present invention, a static type
semiconductor memory device having a thin film semiconductor layer on an
insulator as a main surface and formed on an SOI substrate is provided
which includes a group of transistors, an interlayer insulating layer and
a plurality of load elements. The group of transistors has plural sets of
a pair of access thin film MOS transistors and a pair of driving bipolar
transistors of a lateral type formed at the thin film semiconductor layer.
The interlayer insulating layer covers a surface of the transistor group.
Load elements form a pair corresponding to each pair of driving bipolar
transistors of a lateral type, and are formed on the interlayer insulating
layer and connected to the driving bipolar transistors of a lateral type
through contact holes formed at the interlayer insulating layer. The load
element has one end connected to a first power supply and another end
connected to a collector of the driving bipolar transistor of a lateral
type, which has an emitter connected to a second power supply, so that the
load element and the corresponding bipolar transistor of a lateral type
form an inverter. The pair of driving bipolar transistors of a lateral
type have collectors and bases which are cross-connected to each other,
and base regions each having a common region shared with one of source and
drain regions of the access thin film MOS transistor. The collector region
of the driving bipolar transistor of a lateral type is formed by a region
of the same conductivity type extending from a surface of the thin film
semiconductor layer to an interface with the insulator. A contact hole is
open to the collector region of one transistor and to the base region of
the other transistor of the pair of driving bipolar transistors of a
lateral type.
Therefore, an advantage of the present invention is that low voltage
operations can be achieved, since the driving transistor of an inverter
forming a memory cell is a bipolar transistor and thus the logic threshold
of the inverter is determined by a threshold voltage of the bipolar
transistor.
Another advantage of the present invention is that since the access MOS
transistor and the driving bipopolar transistor of a lateral type forming
the memory cell have a common region shared by the source/drain region and
the base region, reduction in memory cell area can be achieved as compared
with the case where these regions are formed separately.
A still another advantage of the present invention is that reduction in
memory cell area can be achieved, since the access transistor and the
driving bipolar transistor are formed at the thin film semiconductor layer
on the insulator and element isolation of the pair of driving bipolar
transistors are effected by the collector region.
A further advantage of the present invention is that reduction in memory
cell area can be achieved with a smaller number of contact holes, since
the collector region of one transistor and the base region of the other
transistor of the pair of driving bipolar transistors of a lateral type
forming a memory cell are connected by an interconnection layer formed at
the contact hole which is open to both of these regions.
The foregoing and other objects, features, aspects and advantages of the
present invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram showing an equivalent circuit of a memory cell
in a static type semiconductor memory device according to a first
embodiment of the present invention.
FIGS. 2(a) and 2(b) are a circuit diagram showing a second equivalent
circuit of the memory cell according to the first embodiment of the
present invention, in which (a) shows an equivalent circuit of a first
inverter, and (b) shows an equivalent circuit of a second inverter.
FIG. 3 is a graph for use in illustration of operations, showing transfer
characteristics of the inverter forming the memory cell according to the
first embodiment of the present invention.
FIG. 4 is a graph for use in illustration of operations, showing transfer
characteristics of the memory cell according to the first embodiment of
the present invention.
FIGS. 5a-5c are a plan view showing patterns of a memory cell according to
the second embodiment of the present invention in which (a) shows a
pattern of a bottom layer, (b) shows a pattern of an intermediate layer,
and (c) shows a pattern of an upper layer.
FIGS. 6a-6b are a cross sectional view showing cross-sectional structures
of the second embodiment of the present invention in which (a) shows the
cross section taken along the line 6(a)--6(a) in FIG. 5 and (b) shows a
cross section taken along the line 6(b)--6(b) in FIG. 5.
FIGS. 7a-7e are a cross sectional view showing the flow of the steps for
manufacturing the memory cell according to the second embodiment of the
present invention in which (a) through (e) show the cross sections in the
first through fifth steps, respectively.
FIGS. 8-8i are a cross sectional view showing the flow of the steps for
manufacturing the memory cell according to the second embodiment of the
present invention in which (f) through (i) show the cross sections in the
sixth through ninth steps, respectively.
FIG. 9 is a circuit diagram showing an equivalent circuit of a memory cell
according to a third embodiment of the present invention.
FIGS. 10a-10d are a plan view showing patterns of the memory cell according
to the third embodiment of the present invention in which (a) through (d)
show patterns of the layers of first through fourth groups, respectively.
FIGS. 11a-11b are a cross sectional view showing cross sectional structures
of the memory cell according to the third embodiment of the present
invention in which (a) shows a cross section taken along the line
11(a)--11(a) in FIG. 10 and (b) shows a cross section taken along the line
11(b)--11(b) in FIG. 10.
FIGS. 12a-12b are a cross sectional view showing cross sectional structures
of a memory cell according to a fourth embodiment of the present invention
in which (a) shows a cross section taken along the line 11(a)--11(a) in
FIG. 10 and (b) shows a cross section taken along the line 11(b)--11(b) in
FIG. 10.
FIG. 13 is a cross sectional view showing a parasitic bipolar transistor in
the second embodiment of the present invention.
FIG. 14 is a circuit diagram showing an equivalent circuit including the
parasitic bipolar transistor.
FIGS. 15 and 16 are cross sectional views showing structures of memory
cells according to fifth and sixth embodiments of the present invention,
respectively.
FIGS. 17a-17c are a plan view showing patterns of a memory cell according
to a seventh embodiment of the present invention in which (a) to (c) show
patterns of a bottom layer, an intermediate layer, and an upper layer,
respectively.
FIGS. 18a-18b are a cross sectional view showing cross sectional structures
of the memory cell according to the seventh embodiment of the present
invention in which (a) shows a cross section taken along the line
18(a)--18(a) in FIG. 17 and (b) shows a cross section taken along the line
18(b)--18(b) in FIG. 17.
FIG. 19 is a circuit diagram showing an equivalent circuit of a highly
resistive load type memory cell in a first conventional example.
FIG. 20 is a circuit diagram showing an equivalent circuit of a CMOS load
type memory cell in the first conventional example.
FIGS. 21a-21c are a plan view showing patterns of the resistive load type
memory cell according to the first conventional example in which (a) to
(c) show patterns of a bottom layer, an intermediate layer, and an upper
layer, respectively.
FIGS. 22a-22b are a cross sectional view showing cross sectional structures
of the resistive load type memory cell according to the first conventional
example in which (a) and (b) show cross sections taken along the lines
22(a)--22(a) and B--B' in FIG. 21, respectively.
FIGS. 23(a)--23(b) illustrate operations, showing curves of data readout
characteristics of a conventional memory cell and (b) shows an equivalent
circuit upon readout.
FIG. 24 is a circuit diagram showing an equivalent circuit of a second
conventional example.
FIG. 25 is a second equivalent circuit diagram of the second example in
which (a) shows an equivalent circuit of a first inverter, and (b) shows
an equivalent circuit of a second inverter.
FIG. 26 is a graph for use in illustration of operation, showing curves of
data readout characteristics of a memory cell in the second conventional
example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
FIG. 1 is a circuit diagram showing a structure of a memory cell of a
static type semiconductor memory device according to the first embodiment
of the present invention.
This structure is different from that of the second conventional example in
that driver MOSFETs Q1 and Q2 are replaced with NPN bipolar transistors T1
and T2.
More specifically, highly resistive elements R1 and R2 are connected with
collectors of NPN bipolar transistors T1 and T2, respectively, which have
emitters coupled to a ground potential. NPN bipolar transistors T1 and T2
have their bases and collectors cross-connected with each other. A P type
MOSFET Q3 is connected between bit line BL and a connection point between
highly resistive element R1 and the collector of transistor T1, while a P
type MOSFET Q4 is connected between/bit line /BL and a connection point
between highly resistive element R2 and the collector of transistor T2.
Description will now be made of operations. Since an NPN bipolar transistor
has operation characteristics different from those of an N type MOSFET,
operations of the memory cell will be described with reference to FIGS.
2-4.
Similarly to the second conventional example shown in FIG. 25, the memory
cell is divided into two inverters, as shown in FIG. 2. It is appreciated
that since the bipolar transistor has a base input clamped by a PN
junction diode between the base and the emitter, diodes 14 and 15 are
attached to outputs of respective inverters to be equivalent to the
circuit shown in FIG. 1. Parasitic resistors Rb1 and Rb2 of the bases are
connected between storage nodes A and B and diodes 14 and 15,
respectively, because they cannot be neglected in analyzing the
operations. Base parasitic resistors 16a and 16b, and 17a and 17b are
essentially the same parasitic resistors. However, in dividing the memory
cell into two inverters, each resistor connected to an input of the
inverter has a value obtained by dividing the original resistance value.
FIG. 3 shows transfer characteristics of the inverter shown in FIG. 2(a).
In the graph, the three curves correspond to the cases where V.sub.CC
equals to 1.0 V, 1.5 V, 2.0 V, respectively. When input voltage V(1) of
the inverter is increased and reaches a threshold voltage Vbe of the
bipolar transistor, NPN bipolar transistor T1 attains an ON state, whereby
potential V(2) of the output node changes from the potential of an "H"
level to an "L" level.
FIG. 4 shows transfer characteristics of the inverters shown in FIGS. 2(a)
and (b) overlapped with each other. It can be seen from the graph that two
stable points are obtained as the conventional example, thereby allowing
operation as a memory cell. FIG. 4 shows the case where V.sub.CC is equal
to 1.5 V. The amount corresponding to the "eye of a cell" indicated by the
arrow in FIG. 4 is the difference between threshold voltage V.sub.be of
the bipolar transistor and potential V(2) of storage node A at an "H"
level.
When storage node A is at an "H" level in the readout state, NPN bipolar
transistor T1 is in an OFF state, so that a current flows to the memory
cell from the bit line through access transistor Q3, base parasitic
resistor Rb1 and diode 14. Expressing the value of this current as
I.sub.cell, potential V(2) of storage node A is the potential higher than
clamp potential V.sub.be of diode 14 by the product of current I.sub.cell
and parasitic base resistance Rb. As a result, the amount indicated by the
arrow and corresponding to the "eye of a cell" in FIG. 4 is I.sub.cell
.multidot.Rb.
Thus, in the present embodiment the memory cell can operate under the
following condition:
V.sub.CC >V.sub.be (4)
where V.sub.be is a threshold voltage between the base and the emitter of
the bipolar transistor as described above. Since usually voltage V.sub.be
is approximately 0.7 V , the memory cell can operate even when power
supply voltage V.sub.CC is as low as 1 V .
Second Embodiment
The first embodiment has demonstrated that the memory cell expressed by the
equivalent circuit diagram in FIG. 1 is suitable for low voltage
operations. In the second embodiment, a memory cell structure is provided
which can reduce the area of the memory cell shown in the equivalent
circuit diagram of FIG. 1.
FIG. 5 is a plan view showing patterns according to the second embodiment
of the present invention. The portions, where first polysilicon patterns
32a and 32b are overlapped with the silicon active layer formed at the
surface of the silicon substrate, such as P type regions 31a and 31b, form
channel portions of the active MOS transistors. More specifically, first
polysilicon patterns 32a and 32b correspond to gates of access transistors
Q3 and Q4 in FIG. 1, respectively.
Thus, the first polysilicon patterns 32a and 32b also correspond to word
line WL. The second polysilicon pattern 34b corresponds to a ground
interconnection, while second polysilicon patterns 34a and 34c correspond
to storage nodes A and B of the memory cell, respectively. First buried
contacts 33d and 33c correspond to the contacts for the base regions of
NPN bipolar transistors of a lateral type, connecting second polysilicon
patterns 34c and 34a with the surface of the silicon substrate. First
buried contacts 33b and 33e are emitter contacts of lateral type NPN
bipolar transistors, and first buried contacts 33a and 33f are the
contacts for the collectors of the lateral type NPN bipolar transistors.
The patterns formed at a layer over such patterns will be described with
reference to FIG. 5(b). Third polysilicon patterns 44a and 44b are the
interconnections each connecting the collector of driving NPN bipolar
transistor of lateral type and power supply potential V.sub.CC. By
performing ion implantation with use of mask pattern 46 as a mask,
resistance value of the implanted portion is reduced to serve as an
interconnection and the unimplanted portion serves as a highly resistive
element.
Referring to FIG. 5(c), as the patterns formed at a further upper layer,
metal interconnections 36a and 36b correspond to bit line pair BL and /BL
in FIG. 1 and connect to the surface of the silicon substrate through
contact holes 35a and 35b.
Therefore, this structure is different from that of the first conventional
example shown in FIG. 21 in that the patterns of active regions 31a and
31b are changed because the bipolar transistor is used as a driving
element of the memory cell and that second polysilicon patterns 34a-34c
connecting these patterns are changed.
FIG. 6 is a cross sectional view in which (a) and (b) show cross sections
taken along the lines A-A' and B-B' in FIG. 5, respectively. On silicon
substrate 51, field oxide film 52 for element isolation is formed. The
portion at the surface of silicon substrate 51 which is not covered by
field oxide film 52 functions as the silicon active layer. Region 54b of
the P.sup.+ type source/drain region is integrally formed with the base
region of the bipolar transistor. An N type silicon layer 57a is formed by
ion implantation with use of first buried contact pattern 33e as a mask
and corresponds to the emitter region of the NPN bipolar transistor. An N
type silicon layer 57b is formed by ion implantation with use of first
buried contact pattern 33f as a mask and corresponds to the collector
region of the NPN bipolar transistor.
First polysilicon patterns 32a and 32b each form an MOS type transistor at
the surface of the silicon active layer. More specially, in FIG. 6(a),
first polysilicon pattern 32a forms the gate of the P type MOSFET and
first polysilicon pattern 32b is located on field oxide film 52 in the
cross section A-A', serving merely as an interconnection.
Second polysilicon patterns 34a, 34b and 34c are connected to the surface
of silicon substrate 51 through first buried contacts 33d, 33e and 33f.
Metal interconnection 36a is connected to the surface of silicon substrate
51 through contact hole 35a.
Since FIG. 6(b) is the mirrored version of (a) of FIG. 6 and corresponding
portions are identical, description thereof will not be repeated.
FIGS. 7(a)-(e) and FIGS. 8(f)-(i) are cross sectional views showing the
flow of the steps of manufacturing the memory cell having the cross
sectional structure shown in FIG. 6.
First, a surface of an N type silicon substrate is oxidized to form an
SiO.sub.2 film 10. An Si.sub.3 N.sub.4 film 12 used as a mask for
selective oxidation is formed by CVD method. Thereafter, photoresist for
forming an element isolation pattern is prepared, the Si.sub.3 N.sub.4
/SiO.sub.2 film is etched, and the photoresist is removed (FIG. 7(a)).
Next, selective oxidation is performed by using the Si.sub.3 N.sub.4
/SiO.sub.2 film as a mask to form field oxide film 52. The Si.sub.3
N.sub.4 /SiO.sub.2 film 12 used as a mask for selective oxidation is
removed (FIG. 7(b)).
Subsequently, a gate oxide film (not shown) is formed at the surface after
the SiO.sub.2 film is removed. A first polysilicon layer is formed and
first polysilicon patterns 32a and 32b are formed by employing the mask,
formed by photolithography, and dry etching (FIG. 7(c)).
The P type active regions 54a and 54b are formed by implanting ions of P
type impurity with use of first polysilicon pattern 32a and field oxide
film 52 as a mask followed by annealing. Although the P type impurity to
be implanted is not particularly limited, by using boron (B) or the like a
P type impurity region with a depth of about 0.3 .mu.m is formed. As will
be described later, P type impurity regions thus formed correspond to the
source and drain regions of the access transistor and the base region of
the lateral type bipolar transistor (FIG. 7(d)).
After interlayer insulating film 40 is formed, contact holes 33d, 33e and
33f are formed by employing the mask prepared by photolithography and dry
etching. By using contact hole patterns 33e and 33f as a mask, ions of N
type impurity are selectively implanted. After activated by annealing, N
type active regions are formed corresponding to emitter region 57a and
collector region 57b of a lateral type NPN bipolar transistor (FIG. 7(e)).
It should be noted that annealing of the P type impurity region in FIG.
7(d) can be performed simultaneously with annealing of the N type impurity
region described above.
Although the N type impurity to be implanted is not particularly limited,
by using arsenic (As) or the like an N type impurity region with the depth
of about 0.15 .mu.m can be formed.
A second polysilicon layer is formed, and second polysilicon patterns 34a,
34b and 34c are formed by using the mask prepared by photolithography and
dry etching. An interlayer insulating film is formed again, followed by
formation of second contact hole 39a (FIG. 8(f)).
A third polysilicon layer is deposited and etched to form third polysilicon
patterns 44a and 44b (FIG. 8(g)).
By using mask pattern 46 as a mask, ion implantation is performed to
selectively form low resistance region 69 and high resistance region 70
(FIG. 8(h)).
After contact hole 35a is formed and metal interconnection layer 36 is
deposited, metal interconnection patterns 36a and 36b are formed (FIG.
8(i)).
Through the steps above, a memory cell is formed which has a P type MOSFET,
resistor 70 and a lateral type NPN bipolar transistor having N type region
57a as an emitter region, N type region 57b as a collector region, and a P
type region sandwiched by emitter region 57a and collector region 57b as
an intrinsic base region.
By adopting the steps above, base region 54b of the lateral type NPN
bipolar transistor is integrally formed with the source/drain region of
the P type MOSFET and emitter region 57a and collector region 57b are
formed at the surface of the substrate in the base region, so that a
memory cell including a bipolar transistor and a P type MOSFET can be
formed with a reduced area.
Although in the steps above the ions of N type impurity are implanted
simultaneously to emitter region 57a and collector region 57b as shown in
FIG. 7(e), the present embodiment is not limited thereto. More
specifically, ions of N type impurity can be implanted separately to
emitter region 57a and collector region 57b so that optimum concentrations
of the impurity and the optimum implanted-impurity profiles can be
obtained at respective regions. In addition, the doping method of N type
impurity is not limited to the ion implantation method as described above.
Emitter region 57a and collector region 57b can be doped by thermal
diffusion from the polysilicon pattern directly contacting these regions.
Third Embodiment
Description has been made in the second embodiment of the memory cell,
having a highly resistive element as a load and a lateral type NPN bipolar
transistor as a driving element, as a memory cell in the static type
semiconductor memory device.
In the third embodiment, thin film transistors Q5 and Q6 are utilized as
load elements in the second embodiment.
FIG. 9 is a circuit diagram showing an equivalent circuit of a memory cell
according to the third embodiment. The present embodiment is different
from the second embodiment in that thin film transistors Q5 and Q6 are
employed as load elements connected to the collectors of NPN bipolar
transistors T1 and T2, as described above.
FIGS. 10(a)-(d) show patterns of the memory cell in the static type
semiconductor memory device according to the third embodiment, divided
into several layers as in the first conventional example.
The present embodiment is different from the second embodiment in the
following two points.
First, gates of thin film transistors Q5 and Q6 are formed by third
polysilicon patterns 38a and 38b. The gates of thin film transistors Q5
and Q6 are connected to base regions of the lateral type NPN bipolar
transistors through contact holes 39a and 39b.
Secondly, the regions of fourth polysilicon patterns 44a and 44b which are
implanted with ions by using mask pattern 46 as a mask form source/drain
regions of thin film transistors Q5 and Q6, and the unimplanted regions
form channel regions of thin film transistors Q5 and Q6. Since the rest of
the structure is similar to the second embodiment, the identical portions
are labeled with the identical reference numerals and description thereof
will not be repeated.
FIG. 11 is a cross sectional view in which (a) and (b) show cross sections
taken along the lines A-A' and B-B' in FIG. 10. Third polysilicon patterns
38a and 38b are the gate patterns of thin film transistors Q5 and Q6. Of
fourth polysilicon patterns 44a and 44b formed on gate patterns 38a and
38b with an insulating film interposed therebetween, regions 70a and 70b
which are not implanted with ions are opposite to gate pattern 38a and 38b
to form channel regions of thin film transistors Q5 and Q6.
As described above, with the structure in which the load elements are
formed by thin film transistors Q5 and Q6, reduction in area of a memory
cell can be achieved by integrally forming the base region of the lateral
type NPN bipolar transistor and the source/drain region of the P type
MOSFET.
Fourth Embodiment
FIG. 12 is a cross sectional view in which (a) and (b) show cross sectional
structures of a memory cell in a static type semiconductor memory device
according to a fourth embodiment of the present invention. Since only the
active elements (the driving bipolar transistor and the access MOS
transistor) at the surface of the silicon substrate are changed from the
conventional example, load elements are not shown. The structure of the
load element can be implemented similarly to the conventional example.
The cross section shown in FIG. 12(a) corresponds to the A-A' cross section
in FIG. 5 and the cross section shown in FIG. 12(b) corresponds to the
B-B' cross section in FIG. 5. In the third embodiment, the impurity
concentration or the depth of the base region of the driving NPN bipolar
transistor of lateral type is the same as that of the P.sup.+ type
source/drain regions of access transistors Q3 and Q4.
In the fourth embodiment, the step of implanting ions to P.sup.+ type
source/drain regions 54a and 54b of access transistors Q3 and Q4 are
performed separately from that to base region 59 of the driving NPN
bipolar transistor of lateral type.
Structured as such, junction depth and concentration of the base region of
the bipolar transistor can be determined without an increase in area of
the memory cell, thereby allowing optimization of parameters of the
bipolar transistor.
For example, high impurity concentration is desired at the source/drain
regions of access transistors Q3 and Q4, while high impurity concentration
causes deterioration in implantation efficiency at the base region of the
bipolar transistor. Thus, it is required to set the concentrations of
these areas independently.
Fifth Embodiment
Since a P type MOSFET is employed as an access transistor in the second
through fourth embodiments, the silicon substrate under the base region of
the bipolar transistor is of N type and the N type substrate is usually
connected to power supply potential V.sub.CC. FIG. 13 shows a cross
sectional structure of the bipolar transistor portion according to the
second embodiment. As described above, underlying the P type base region
54b of the bipolar transistor is the N type substrate which is connected
to power supply potential V.sub.CC. As a result, a parasitic bipolar
transistor P1 is present in addition to the original lateral type bipolar
transistor T1. More specifically, there exists a parasitic bipolar
transistor which shares the emitter region and the base region with
lateral type bipolar transistor T1 and has the N type substrate as the
collector region.
For lateral type bipolar transistor T2 as well, a similar parasitic bipolar
transistor P2 is present.
FIG. 14 is a circuit diagram showing an equivalent circuit of a memory cell
including parasitic bipolar transistors P1 and P2.
Parasitic bipolar transistors P1 and P2 share the emitter and the base with
lateral type bipolar transistors T1 and T2 respectively because of the
structure, whereby the minority carriers introduced from the emitter to
the base flow in great amount to the collectors of parasitic bipolar
transistors P1 and P2. As a result, current amplification ratio (h.sub.fe)
of bipolar transistors T1 and T2 appears to be decreased, thereby causing
deterioration in performance of lateral type bipolar transistors T1 and
T2.
FIG. 15 shows a cross sectional structure of a memory cell in a static type
semiconductor memory device according to a fifth embodiment of the present
invention. It is different from the fourth embodiment shown in FIG. 12 in
that a high concentration region 60 of the same conductivity type as the
base region is formed in the semiconductor substrate under base region 59.
Such high concentration layer can be formed by carrying out ion
implantation for forming the base layer a plurality of times with
different acceleration energies and different amounts of ions to be
implanted.
By thus forming the high concentration layer, transport efficiency of
parasitic bipolar transistors P1 and P2 are decreased, thereby solving the
problems of deterioration in characteristics of lateral type bipolar
transistors T1 and T2 due to operations of the parasitic bipolar
transistor elements, as described above.
Sixth Embodiment
FIG. 16 shows a cross sectional structure of a memory cell in a static type
semiconductor memory device according to the sixth embodiment of the
present invention. It is different from the fifth embodiment in that the
high concentration layer in the fifth embodiment is replaced with an
insulating barrier layer 61. An example of such barrier layer is a buried
oxide layer. The buried barrier layer of an oxide film can be formed by
implanting ions of oxygen more deeply into the substrate for forming the
base region as described in the fourth embodiment, followed by annealing.
Barrier layer 61 allows reduction in transport efficiency of the parasitic
bipolar transistor as described in the fifth embodiment and allows to
obtains the effects similar to the fifth embodiment.
Seventh Embodiment
FIG. 17 is a plan view showing patterns of a memory cell in a static type
semiconductor memory device according to the seventh embodiment of the
present invention.
The present embodiment is different from the second embodiment in that the
elements forming the circuit shown in FIG. 1 is formed on a substrate
having a thin film semiconductor layer on an insulator, i.e. an SOI
(Silicon on Insulator) substrate. In addition, the cross-connection of the
bases and collectors of driving bipolar transistors T1 and T2 are made by
interconnections 34a and 34b formed in contact holes 33a and 33c in the
interlayer insulating layer, which holes are open to collector region 40a
or 40b of one transistor and base region 39c or 39b of the other
transistor.
FIG. 17 shows a plurality of layers divided as illustrated in (a), (b) and
(c). FIG. 17(a) shows an active region at the thin film silicon layer
formed on the insulator. Regions 39a, 39b, 39c and 39d are P type regions,
corresponding to the source/drain regions of access transistors Q3 and Q4
and base regions 39b and 39c of the lateral type bipolar transistors.
Regions 40a and 40b are N type regions, corresponding to the collector
regions of the lateral type bipolar transistors. Regions 47a and 47b are
also N type regions, corresponding to the emitter regions of the lateral
type bipolar transistors. Common region 39c shared by the base region and
source/drain region is connected to collector region 40a at buried contact
33c portion. Similarly, common region 39b shared by the base region and
the source/drain region is connected to collector region 40b at buried
contact 33a portion. Regions 38a and 38b are the channel regions of access
thin film MOSFETs Q3 and Q4, which are N type regions.
Regarding the patterns formed at a layer over those in FIG. 17(a), first
polysilicon pattern 32 forms a word line as shown in FIG. 17(b).
Interconnection patterns 34a and 34b are connected to the surface of the
thin film semiconductor layer through buried contacts 33a and 33c.
Interconnection patterns 34aand 34b connect base regions 39b and 39c of
the lateral type bipolar transistors with collector regions 40b and 40a,
respectively, as described above. Therefore, interconnection layer 34 is
formed of polysilicon or metal and required to make an ohmic contact with
the SOI surface at both P type and N type regions. Interconnection
patterns 34a and 34b correspond to storage nodes A and B of the memory
cell, and interconnection pattern 34c corresponds to a ground
interconnection. Metal interconnections 36a and 36b form bit line pair BL
and /BL. Metal interconnections 36a and 36b connect to the surface of thin
film silicon layer 37 through contact holes 35a and 35b, respectively.
FIG. 18 is a cross sectional view in which (a) shows a cross section taken
along the line A-A' in FIG. 17 and (b) shows a cross section taken along
the line B-B' in FIG. 17. Referring to FIG. 18, the cross sectional
structure of the seventh embodiment will be described below. On an
insulator substrate 62, thin film semiconductor layer 37 is formed.
Alternatively, if SIMOX (Separated by Implanted Oxygen) technology is
employed, region 62 corresponds to the insulating layer formed near the
surface of the silicon substrate. At thin film silicon layer 37, regions
39a-39d are of P type, and regions 38a, 38b, 40a, 40b, 47a and 47b are of
N type.
First polysilicon layer 32a forms a gate electrode of the access transistor
in the cross section shown in FIG. 18(a). Among interconnection layers
34a, 34b and 34c, especially layer 34b electrically connects P type
silicon at base region 39c with N type silicon at collector region 40a.
Since collector region 40a extends from the surface of thin film
semiconductor layer 37 to insulating region 62, one base region 39b and
the other base region 39c of the driving lateral type bipolar transistors
are separated by PN junction formed by N type region 40a and P type region
39c.
The structure shown in FIG. 18(b) is merely a mirror image of that in FIG.
18(a), and corresponding portions have the identical structures.
With the structure employing an SOI substrate as described above, four
buried contacts 33 are included in a memory cell as shown in FIG. 17,
fewer than six contacts included in the memory cell of the second
embodiment shown in FIG. 5, thereby allowing reduction in area of the
memory cell.
Moreover the current gain of the lateral type bipolar transistor is not
deteriorated by such parasitic bipolar transistor as shown in FIG. 13.
Although the present invention has been described and illustrated in
detail, it is clearly understood that the same is by way of illustration
and example only and is not to be taken by way of limitation, the spirit
and scope of the present invention being limited only by the terms of the
appended claims.
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